1. Technical Field
The present invention relates to a process of manufacturing gas generating material. In particular, the present invention relates to a process of making graphite fibers for use in a material for generating gas to inflate a vehicle occupant restraint such as an air bag.
2. Description of the Prior Art
U.S. Pat. No. 4,999,063 discloses a process for manufacturing a gas generating material. The gas generating material includes graphite fibers for strengthening the bodies of gas generating material made by the disclosed process. The graphite fibers also promote thermal conductivity within the gas generating material.
Uniform distribution of the graphite fibers in the gas generating material is desirable to promote uniform burning of the bodies of gas generating material and to provide uniform strength. Applicant has found that graphite fibers which are relatively long can result in a non-uniform distribution of the graphite fibers in the gas generating material. This non-uniform distribution occurs because during the process of making the gas generating material, longer fibers tend to ball up and trap other fibers. This can result in a relatively large mass of graphite in one location in the gas generating material and no graphite in other locations in the gas generating material.
Prior art processes such as chopping can only shorten graphite fibers to a length of about 1/8 inch. Hammer milling will provide shorter fibers than chopping but does not produce a sufficiently uniform length distribution. The present invention is based on the observation that if the graphite fibers are shorter in length and if the length. distribution of the graphite fibers is as narrow as possible, a more uniform distribution of graphite fibers in the gas generating material will result. This will produce more uniform strength and uniform burning of the bodies of gas generating material.